In a direct-detected optical communications system, multi-path interference (MPI) originates from combinations of reflections of a transmitted waveform at optical interfaces (connectors, receiver/transmitter interfaces). An example is depicted in FIG. 1 which shows a transmitter 100 connected to a receiver 102 over a sequence of optical cables connected at optical interfaces 104,106,108. After transmission of a signal s(t) by transmitter 100, due to reflections at the interfaces 104,106,108, a signal u(t) received at receiver 102 is the sum of a set of delayed replicas of a transmitted signal. Generally indicated at 110 is a logically equivalent version of the system of FIG. 1 showing the summing of four delayed components, with respective attenuations a1, a2, a3, a4.
Another source of the interference is due to reflections that occur in the electrical domain, for example after a photodiode in the receiver, or prior to a laser modulator in the transmitter. These electrical reflections can be cancelled directly, by estimating the delay and amplitude of each component. Such electrical reflections typically occur over very short distances (on the order of tens of mm or less), so they remain within a small number of bauds from the main signal.